I think that if your metric is not total number of universes, but available quantity of causal interactions, then Bostrom's argument falls apart on probability

I think that if your metric is not total number of universes, but available quantity of causal interactions, then Bostrom's argument falls apart on probability as well. More stuff can happen in the top level universe, unless you are dedicating more than half the top level universe to simulation, so you are more likely to exist there.